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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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GLACIERS OF UNAKWIK INLET AND COLLEGE FIORD         295
by glacial erosion. If the low ridge, or swell, is a moraine it is an accumulation interrupting the normal slope of the fiord bottom which descends from a depth of 756 feet south of the Jonah Bay moraine, to 1104 feet at the entrance to Unakwik Inlet.
Four and a half miles north of the Jonah Bay moraine bar, and just west of the mouth of Miner's River, is a third submarine irregularity (O, Fig. 40), interrupting the slope of the fiord bottom, which is 584 feet deep at the end of Meares Glacier and 708 feet near Jonah Bay, with intermediate depths of over 800 feet on either side of the swell in the fiord bottom. The ridge near Miners River rises 800 feet above the fiord bottom to the north, and 246 feet above that to the south; but its crest lies beneath 582 feet of water. As in the last ease, it is difficult to interpret this shoal, which may be either an unremoved rock ridge due to basining of the fiord bottom or a glacial deposit. For the f ollowing reasons, it seems quite possible that it is the latter.
Miner's River, which is the outlet of Boot Lake and a smaller lake below it, is supplied by a stream from Pedro Glacier. At the time when Meares Glacier occupied the whole of Unakwik Inlet, Pedro Glacier was one of its tributaries and, as a much smaller glacier, should not have been able to erode as efficiently as the main ice tongue. In the case of the small glaciers of about the same size as Pedro Glacier, which occupied Jonah Bay and Siwash Bay, the inequality of glacial erosion resulted in these bays being left as submerged hanging valleys. The valley occupied by Miner's River, however, does not now have the hanging valley relationship to the main fiord, for the water is 384 feet deep at the head (Fig. 88), and 489 feet deep in the entrance of the cove, while opposite this cove, in the middle of the main fiord, the depth is only 582 feet. The hills near the outlet of Miner's. River, however, separating Boot Lake from the main fiord seem to be morainic accumulations, and it is quite likely that Boot Lake is a former branch of Unakwik Inlet similar to Siwash Bay but separated from the main bay by morainic deposits. It is therefore considered possible that the Boot Lake valley is a submerged hanging valley of the usual kind, but that the discordance is not revealed because glacial accumulations were made directly opposite the mouth, shallowing the water from 850 feet, or thereabouts, to 582 feet.
Just inside the mouths of Jonah Bay and Siwash Bay are shoals, and Siwash Bay is deeper a mile or two back from the entrance (144 feet), than at its junction with the mfdn fiord, where its depth is only 74 feet. In these cases also it has not been possible to determine whether the shoals are glacial accumulations or rock reefs.
The foregoing statement makes it clear that there is only one large accumulation in Unakwik Inlet which can be actually demonstrated to be, at least in part, a recessional moraine. This is the moraine bar rising nearly or quite to the surface in the main fiord opposite the mouth of Jonah Bay. The shoal to the north, near Miner's River, is possibly a recessional moraine while that to the south is more doubtful. If all three of these are morainic accumulations, however, they furnish interesting evidence of the intervals between halts in Meares Glacier as it retreated northward from Prince William Sound to its present position. It is five miles from the southernmost moraine to the one at Jonah Bay, and 4^ miles from there to the one opposite Miner's River which is 6| miles from the present ice front, perhaps indicating that the glacier halted long enough to build these deposits at rather even intervals.
An alternate explanation of the irregularities in the bottom of Unakwik Inlet might be a rhythmic swing of erosion by a viscous ice body, with (1) a downward plunge at